—6—
The visible and near-infrared domain
Steven V.W. BeckwithI
Abstract
Space observations at visible and near-infrared wavelengths are free from the absorption, high foreground radiation and wavefront distortion caused by the Earth’s
atmosphere. The absence of these effects in space permits observations of higher
sensitivity and stability with better angular resolution and with a larger dynamic
range than their counterparts from the ground. Observations from the HST, the
ISO and Spitzer satellites and survey satellites such as IRAS are still unsurpassed
by ground-based telescopes for their sensitivity, radiometric stability, angular resolution over large fields of view, dynamic range and complete spectral coverage. This
chapter discusses the inherent limitations of ground-based astronomy that are overcome by space observations and lists some of the spacecraft that have demonstrated
these advantages for scientific study.
Introduction
Because the Earth’s atmosphere transmits throughout most of the visible and
infrared domain between 0.35 µm and 25 µm (generally referred to by astronomers
as “optical and infrared” or OIR), astronomy flourished for millennia without access to space. Even in the space age, ground-based telescopes continue to provide us with OIR observations, especially at wavelengths shorter than 2 µm, where
it is possible to make sensitive measurements with large telescopes through the
Earth’s atmosphere. Prior to the development of electronic detectors, space observations of night-sky objects were inefficient and took limited advantage of the lack
of atmosphere from spacecraft. But as detection techniques improved, and computers and electronic imaging detectors combined to replace human observers even
on ground-based telescopes, space-based observations quickly surpassed anything
possible from the Earth’s surface. Today, no ground-based telescope approaches
the combination of natural sensitivity and resolution allowed by observations in
space with equivalent telescope area. The enormous advances made by orbiting
observatories such as HST show that the disadvantages of the atmosphere make it
all but inevitable that many if not all OIR observational techniques will be used
in space at some time, once we develop the means to put very large telescopes in
orbit.
I The
University of California, Oakland CA, USA
113
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6. The visible and near-infrared domain
The Earth’s atmosphere imposes two strong limits on OIR terrestrial observations:
1. The combination of thermal radiation, molecular airglow and scattered light
from the Moon and artificial lighting create foreground emission (normally
called background by astronomers) that provides a fundamental limit to the
sensitivity of astronomical observations from the ground. The salient example
is the difficulty of seeing stars during the daytime owing to the bright sky.
2. Rapidly changing variations of the index of refraction in turbulent air distort
the wavefront of a celestial source, scrambling information about the source
at high spatial frequencies and degrading the natural resolution of a telescope
by one to two orders of magnitude. The wavefront distortions, called seeing,
can be corrected in principle over much of the OIR domain using a technique
called Adaptive Optics (AO), but AO is applicable to a limited range of
observations and does not reliably correct the atmosphere at all wavelengths
nor over a very wide field of view.
The practical difficulties associated with AO are sufficiently daunting that they always compromise the information content of terrestrial observations: AO-corrected
telescopes limit the field of view, dynamic range, radiometric stability, resolution
and astrometric accuracy of an observation from the ground relative to observations in space. These limits are especially severe for observations requiring wide-field
imaging, such as surveys of large regions of the sky.
The combination of detection technology and the development of large rockets
able to launch ever-larger telescopes into orbit drove the evolution of OIR space
telescopes. Increasingly, spectroscopic and interferometric measurements come up
against background limits, as improvements in technology produce instruments
that detect and analyze nearly all of the incident light with no added noise. In fact,
light detectors are currently approaching perfection over most of the OIR domain.
As reflecting optics also perform nearly ideally, increased detection sensitivity will
in future require larger telescope areas.
Space also affords specialized observations such as polarimetric observations
of the Sun with high spatial resolution in the visible. Solar space observations
have given insight into the behaviour of magnetic fields in a stellar atmosphere
(Chapters 19 and 33, Title 2010; Stenflo 2010).
The following sections illustrate how spacecraft with OIR payloads have allowed
modern astronomy to approach the limits of nature.
Background radiation
Background radiation from the atmosphere and zodiacal dust in the vicinity of
the Earth limit the sensitivity of all telescopes. Figure 6.1 shows the rate of photons,
dNγ /dt, seen by ground and space-based telescopes at the highest useful angular
resolution for unresolved sources. An observation that records Nγ photons has
p an
irreducible noise from the statistical fluctuations in the photon stream: σN = Nγ .
The much larger rate of background photons seen by ground-based telescopes means
115
Thermal
1012
106
10
10
10
8
10
6
104
Diffraction-limited
ground-based
Airglow
Ecliptic Pole
1
104
102
Galactic Dust
HlowL
10m
0.5" PSF
MJy sr-1
dNΓ dt Hsec-1 L
102
10-2
10-4
1
10-6
Zodiacal light
10-2
0.3
0.5
1
2 3
5 7 10
Wavelength HΜmL
20 30
50
100
Figure 6.1: Comparison of background photon rates for a 10 m ground-based telescope under ideal conditions, both in the limit of perfect correction for atmospheric
phase distortion and in very good seeing, with a diffraction-limited space telescope
of the same size, in the direction of the ecliptic pole (black line) and with a low
level of galactic dust (red line). At wavelengths longer than ≈ 60 µm, galactic dust
emission becomes an important background source. The scale on the right gives
the background radiance in 1 MJy sr−1 and the bar at the bottom of the graph is
a grayscale representation of the absorption by the terrestrial atmosphere.
that the noise will always be higher than in an equivalent space-based observation
in the background limit, higher by orders of magnitude at wavelengths where thermal emission (285 K) is strong. Spatial as well as rapid temporal variations in the
airglow and sky radiance create an additional source of noise, introducing systematic uncertainties of order Nγ that are much larger than the statistical fluctuations.
The background radiation for space telescopes located within a few million kilometres from Earth is dominated by the zodiacal dust. Sunlight scattered by small
dust particles dominates the background shortward of a few micrometres. At longer
wavelengths the background is caused by thermal emission from dust at temperatures close to those on Earth. However, the thin layer of zodiacal dust near the
Earth has an opacity of ≈ 10−7 , i.e., orders of magnitude smaller than even the
best transmission of the air. Accordingly, the thermal background emission in space
is much smaller than that seen from the ground. The zodiacal emission could be
avoided if a space telescope were situated well above the ecliptic plane or at orbital
distances beyond the asteroid belt, i.e., beyond ≈ 3 ua from the Sun.
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6. The visible and near-infrared domain
Space telescopes operating at wavelengths shorter than a few micrometres take
full advantage of the reduced background even if the optics are at ambient temperatures. For longer wavelengths, the telescope optics must be cooled to reduce
thermal emission from the optical surfaces so that it reaches the zodiacal light limit.
Cooling to below 50 K, such as planned for the JWST, will allow the telescope to
reach the zodiacal light limit up to about 20 µm; observations at longer wavelengths
require greater cooling, typically to less than 10 K, as with ISO or Spitzer.
Wavefront distortion (atmospheric refraction and
“seeing”)
Atmospheric refraction causes the path of a light ray to deviate from a straight
line as it passes through the atmosphere. This is because the air density varies as a
function of altitude. Objects observed from the ground appear to be higher in the
sky than their actual positions, an effect which worsens as the object approaches
the horizon. Atmospheric refraction also disperses the observed radiation for very
high angular resolution studies, with blue wavelengths being more affected than
red wavelengths. Two counter-rotating prisms, Risley prisms, are used to correct
this effect (see, for example, Horch et al 1994).
A more serious problem affecting ground-based observations is the twinkling
of starlight, caused by the disturbance of the wavefront when the light passes
through the Earth’s atmosphere. Turbulence in the air creates rapid variations in
the index of refraction, inducing differential phase delays of several micrometres
across distances of a few decimetres perpendicular to the direction of the incident
light rays. These phase differences create a corrugated wavefront from an initially
plane wave and introduce variations in the wave amplitude as well. The corrugated
wavefront spreads the image at the focal plane of a telescope, producing a pattern
of speckles over an area with a diameter of ≈ 1′′ , with the size of each speckle being
approximately equal to the diffraction-limited point-spread function (PSF) of the
telescope. The speckle pattern changes on a timescale of 10 ms at wavelengths in
the visible part of the spectrum (Hardy 1998).
Figure 6.2 shows a short exposure image of the double star, ζ Booetis, revealing
the speckle pattern. The inset shows the resolution that would be achieved in space
with the same telescope. This image is typical of ground-based images without any
correction for atmospheric distortion. The impact of the atmosphere is wavelength
dependent. The wavefront distortion comes about from approximately fixed time
delays in the arrival of the wave at different points across the wavefront, but the
phase delay varies nearly inversely with the wavelength. The correlation length of
the phase, r0 , varies with wavelength as λ6/5 . At a wavelength of 10 µm, a small
telescope like the one used for the image in Figure 6.2 would be nearly diffractionlimited with single speckle that moved around in the focal plane due to changes
in the tilt of the wavefront: the correlation length r0 (λ = 10 µm) is almost five
times larger than the diameter of the telescope, Dtel /r0 ≈ 0.2, and the wavefront
is well-correlated across the pupil.
One of the best ways to illustrate the impact of a distorted wavefront on the
image is using the Strehl ratio. The Strehl ratio is defined as the maximum intensity
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Figure 6.2: The image (black on white) of the double star ζ Booetis taken around
λ = 550 nm on the 2.6 m Nordic Optical Telescope shows identical speckle patterns
for the two stars separated by 0.8′′ . Each set of speckles is spread out over a region
approximately 0.4′′ across (r0 ≈ 0.4 m). The inset pictures (white, with a falsecolour grey scale, on black) show how the stars would look with the same telescope
at its diffraction limit.
in an image divided by the maximum intensity in a diffraction-limited image, and it
is normally used as a measure for long-exposure images where the speckle pattern
is averaged to produce an approximately Gaussian-shaped PSF with a width equal
to the so-called seeing angle. The Strehl ratio of the image in Figure 6.2 is about
0.017 (Dtel /r0 ≈ 7). In the limit r0 ≪ Dtel , the Strehl ratio is approximately equal
to (r0 /Dtel )2 .
Figure 6.3 shows how the Strehl ratio varies with wavelength for three different
ground-based telescopes: a 2.4 m and a 10 m telescope under good seeing (0.5′′ ),
and a 10 m telescope with an adaptive optics system that reduces the RMS wavefront errors to 0.2 µm, the projected state of the art within 10 years. The Strehl
ratio for uncompensated telescopes under excellent conditions falls below 0.1 at
wavelengths of about 2 µm and 5 µm, respectively, for the first two ground-based
telescopes mentioned above. Even the best AO corrected telescope has Strehl ratios
substantially smaller than 1 at all visible wavelengths. Since a space telescope will
generally have a Strehl ratio equal to 1 at all wavelengths, the advantage of space
for imaging is apparent from this figure. By comparing Figure 6.3 with Figure 6.1,
one concludes that space telescopes have an advantage even if AO techniques can
remove most of the atmospheric distortion: AO correction works best at infrared
wavelengths where, however, the atmospheric background is high. Only in a narrow
window between about 1 µm and 2 µm can an AO compensated telescope achieve a
sensitivity comparable to a space telescope, and then only when the ground-based
telescope has a substantially larger aperture. In practice, the use of large groundbased telescopes brings an advantage only to observations that require very high
spectral resolution — a situation where the background rates are reduced.
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6. The visible and near-infrared domain
1
Strehl ratio
0.3
0.1
0.03
0.01
0.003
0.001
0.1
0.2 0.3 0.5
1
2 3
ΛHΜmL
5
10
20 30
Figure 6.3: The Strehl ratio (peak radiance in the real image of a point source
relative to the peak radiance in the diffraction-limited case) variations for 2.4 m
(solid) and 10 m (dashed) telescopes in 0.5′′ seeing and an AO-corrected groundbased 10 m telescope with residual wavefront errors of 200 nm (short-long dash),
i.e., the best projected performance ten years from now. The 2.4 m HST achieves
a Strehl ratio of 1 throughout this wavelength range.
Sources of radiation
Starlight is the primary source of radiation in the OIR domain. The integrated
visible and infrared light from the Milky Way and more distant galaxies produces
a global irradiance second only to that of the cosmic microwave radiation in importance. Figure 6.4 shows the relative contributions of different energy sources over
the entire spectrum. Stellar spectra are approximately blackbodies with narrow
lines from ions, atoms, and molecules superimposed, the relative distribution and
line strengths depending on the effective temperature and composition of the stellar
atmosphere. The effective temperatures of normal stars range from approximately
(2000 to 40 000) K, resulting in maximum flux densities between about 0.1 µm and
2 µm. The Sun has an effective temperature of ≈ 5780 K, its spectral irradiance
therefore peaks near 0.5 µm, if expressed as Lλ (i.e., in units of W m−2 nm−1 ) or
near 0.9 µm, if expressed as Lν (i.e., in units of W m−2 Hz−1 ) . For regions of the
sky where starlight is the dominant energy source, the OIR range contains the
majority of the radiation and is almost always critical to studying the physical
conditions of the matter.
In addition to thermal radiation from stars, there are two other important
sources of continuous radiation, namely synchrotron radiation from ionized gas in
the presence of magnetic fields and bremsstrahlung from ionized gas in H II regions,
typically referred to as free-free radiation. Almost all synchrotron sources that can
be observed in the OIR domain are near the low-frequency limit; their flux density
is proportional to the −1/3 power of wavelength (Ginzburg and Syrovatskii 1965).
Free-free radiation from a plasma at temperature T varies as exp[−h c0 /(λ kB T )],
119
10-6
ΝIΝ HW m-2 sr-1 L
10
Cosmic Background
-7
Milky Way
10-8
10-9
10-10
10-11
10-12
10-13
1Þ
0.01
1Μm
100
Wavelength
1cm
Figure 6.4: The average global spectral radiance (multiplied by the frequency) in the
solar neighborhood (from Wright 2008). The cosmic background (black) and Milky
Way (red) lines show the separate contributions from extragalactic and galactic
sources. Data from Gruber et al (1999), Madau and Pozzetti (2000), Hauser and
Dwek (2001), Franceschini et al (1997), Stanev and Franceschini (1998), Bernstein
et al (2002) and Wright (1996).
with T approximately 104 K for H II regions, but is often several times hotter in
planetary nebulae and other ionized regions (Allen and Cox 2000).
Transitions between the electronic energy levels of most atoms and molecules
have energies of a few electronvolts, corresponding to wavelengths of a few hundred nanometres, squarely within the OIR domain. Molecular bond-strengths are
between a few hundred millielectronvolts and a few electronvolts, placing the vibrational transitions in the infrared part of the OIR region. With the exception of
molecular hydrogen, pure rotational transitions of molecules are in the radio spectrum. Spectroscopic observations in the OIR include atomic and molecular lines
from stars, from gas in circumstellar regions, and from gas in the interstellar medium (ISM); they occur in emission and absorption. Table 6.1 lists the wavelengths
of a small sample of atomic and molecular lines that are frequently used to study
the physical conditions in the interstellar medium.
Small solid particles — interstellar dust — can resonantly absorb and emit light
over relatively narrow wavelength ranges. These resonant features arise from vibrational excitations of atoms and radicals bound to the surfaces of the particles, and
have fractional bandwidths, ∆λ/λ, of order 0.1, i.e., they are much wider than the
lines from discrete energy transitions in single molecules but nevertheless considerably narrower than continuous sources such as thermal radiation. Many of these
features are useful to understanding the size and composition of interstellar and
circumstellar dust (Spitzer 1978; Sellgren 1984; Léger and Puget 1984). The most
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6. The visible and near-infrared domain
Table 6.1: Selected interstellar lines.
Nebular emission
Atom / ion λ/nm
Ion
H
Na
+
O
O++
N+
434.0 (Hγ )
486.2 (Hβ )
656.2 (Hα )
1281.8 (Pβ )
1875.1 (Pα )
2165.5 (Bγ
2625.2 (Bβ )
4051.2 (Bα )
372.6; 372.9
495.9
500.7
654.8
658.4
a
K
Ca
Ca+
Ti+
Fe
ISM absorption
λ/nm
Molecule
330.2; 330.3
589.0; 589.6
766.5
769.9
422.7
393.4
396.9
307.3
322.9
324.2
338.4
372.0
386.0
CH
CN
CH+
C13 H+
λ/nm
313.8
314.3; 314.6
387.9
388.6; 389.0
430.0
387.4–7a
344.7
357.5
374.5
395.8
423.4
423.2
several lines within the range indicated
salient features at ultraviolet wavelengths are indicative of graphite (Mathis 1990)
and several complexes at 3.3 µm and 7.7 µm that arise from Polycyclic Aromatic
Hydrocarbons, PAHs (Puget and Léger 1989).
Dust absorbs light along the line of sight, but it can also scatter light. Absorption
and scattering are selective in wavelength. Scattering occurs most strongly for short
wavelengths owing to the small size of the dust particles (van de Hulst 1981).
Scattered light thus becomes bluer and transmitted light becomes redder relative
to its source. As such, the colour of radiation gives information about interstellar
dust, as well as its original source, usually stars.
In summary, then, space observations have several advantages that bring qualitative changes to traditional astronomical methods: freedom from atmospheric
absorption gives access to observations covering large, continuous wavelength regions; the very high signal stability permits the detection of tiny time variations;
the high contrast — virtually unattenuated by the intervening medium — lets us
study faint light in the immediate vicinity of bright stars and quasars; moreover, a
stable diffraction-limited PSF allows the study of individual stars in distant galaxies and enables an extraordinary astrometric accuracy on distant stars and clusters
of stars (Chapter 16, Lindegren 2010). And finally, space observations have very
high sensitivity at red and infrared wavelengths, where the terrestrial backgrounds
are high.
Freedom from atmospheric absorption also means that the entire spectrum of a
star, including the Sun (Chapter 32, Fröhlich 2010), is available to measure bolometric luminosity directly. Moreover, all lines are accessible for sources at any
redshift. This is an important advantage for deriving samples of galaxies at cosmological distances; ground-based samples normally have gaps in redshift intervals
where the prominent lines are blocked by the Earth’s atmosphere. Ultraviolet lines
of high-redshift objects, such as quasars, may be seen through the Earth’s atmo-
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sphere in the visible and infrared, but in low-redshift objects these lines can only be
seen with space telescopes. Furthermore, lines from molecules like H2 O, CO2 , and
OH that are strong in the atmosphere are easily seen from space but unobservable
from the ground.
The increased sensitivity of space telescopes at near-infrared wavelengths has
also been important to study objects such as supernovae at high redshifts, where
the locally emitted blue light is in the infrared (Riess et al 2004). The deepest
images of the Universe come from the 2.4 m HST (Williams et al 1996; Beckwith
et al 2006): the reduced background and small PSF provide deeper images than
can be made with even the largest telescopes on the ground.
The stable diffraction-limited PSF of the HST also made it possible to study
individual Cepheid stars in Virgo-cluster galaxies, providing a measure of the
Hubble constant with sufficient accuracy to settle the long running controversy
about the age of the Universe (Freedman et al 2001). The same advantage allows
observations of solar luminosity stars in M31 to derive the ages of the several
populations that make up that galaxy by observing the so-called main-sequence
turnoff in the Hertzsprung–Russell diagram (Brown et al 2004). The combination of
a small PSF with high spectral resolution provided the first unambiguous rotation
curve in the immediate vicinity of the nucleus of a galaxy, confirming that only
a Black Hole had sufficient mass density to explain the high velocities of the
gas (van der Marel et al 1997). Moreover, the high radiometric stability of space
allowed the detection of an atmosphere around the extrasolar planet HD 209458b
using differential spectroscopy on the light curve, as the planet transited the face of
its host star (Charbonneau et al 2002) — one of the more impressive observations in
modern astronomy. Finally, the smallest exoplanet COROT-Exo-7b, with a radius
of 1.6 Earth radii, which has been concluded to be a truly Earth-like, rocky planet,
was discovered by the COROT satellite.
Selected space missions
Optical and infrared missions entered the space age only after the first missions
explored wavelengths utterly unobservable from the ground, notably at X-ray and
ultraviolet wavelengths. Since most of the OIR region could be observed from the
ground — and because infrared detectors were less developed (or information about
them was classified) — there was less pressure from the OIR community to take on
the challenges and expense of space. But the enormous advantages of continuous
transmission, reduced background radiation, and freedom from atmospheric wavefront distortion inevitably drew OIR observers to create space observatories. The
resulting missions produced a rapid expansion of our knowledge about the Universe
and at the same time increased the popularity of astronomy and space science. Images of the Cosmos taken by the HST and images from the surface of Mars and
Titan returned by probes from the surfaces of these planets created a public engagement in the space programme rivaled only by the initial Moon landings of the
Apollo programme.
The National Space Science Data Center (NSSDC) at NASA’s Goddard Space
Flight Center provides a useful reference for space missions that can be found at
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6. The visible and near-infrared domain
http://nssdc.gsfc.nasa.gov/. The following subsections give a brief overview of the
missions that covered the visual/near-infrared bands.
IRAS
–
–
–
–
–
–
–
Launch: 25 January 1983, Delta from Vandenberg AFB, USA
Orbit: 900 km LEO
Optical system: 0.6 m Ritchey–Chrétien telescope, cooled by liquid helium
Wavelength range: 8 µm to 120 µm
Instruments: four-band photometry, low-resolution spectrometer
Mass: 1100 kg
Lifetime: 10 months
The IRAS was the first Explorer-class satellite designed to survey the entire
sky at infrared wavelengths between 8 µm and 120 µm. IRAS was a joint mission
between the United States (NASA), the Netherlands, and the United Kingdom.
IRAS contained a 0.6 m Ritchey–Chrétien telescope cryogenically cooled with
liquid helium below 10 K. An array of 62 detectors in the focal plane covered
four broad wavelength bands centred at (12, 25, 60, and 100) µm. The survey was
carried out by rotating the satellite at a constant angular velocity perpendicular
to the satellite-Sun vector, and detecting sources as they transited the fixed array
of detectors in the focal plane.
IRAS detected approximately 350 000 sources including many that had wellknown positions but had never been seen before in the infrared. Source positions
were accurate to about 30′′ , providing a rich catalogue that is still used as a reference for infrared irradiances of stars and galaxies. Following the sky survey, IRAS
carried out pointed observations, where a low-resolution spectrometer provided
supplementary spectra for many of the more interesting sources, until the cryogen
was depleted on 21 November 1983.
Details of the IRAS mission and its first scientific results were published by
Neugebauer et al (1984), and by further authors in the 1 March 1984 issue of
Astrophysical Journal Letters.
Hipparcos
–
–
–
–
–
Launch: 8 August 1989, Ariane 44LP from Kourou, French Guiana
Orbit: 507 km to 35 888 km elliptical
Optical system: 0.3 m Schmidt telescope
Mass: 1025.0 kg
Lifetime: 3.5 a
Hipparcos took advantage of the wavefront stability above the atmosphere to
measure the astrometric positions of 120 000 stars for parallaxes, proper motions
and positions with an accuracy of 2 mas. Two fields of view 58◦ apart were imaged
through a single telescope onto a focal plane, which consisted of a regular grid of
2688 transparent parallel slits. The spacecraft spun slowly around an axis at 12
revolutions per day to scan the stars across the focal plane. The grid modulated
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the intensity of light from the stars which was detected by a photon-counting image
tube to detect the phase difference of the modulated light from the two separated
fields, thus providing relative positions of stars in the two fields to high accuracy.
The large angular separation of the two fields of view reduced the systematic uncertainties that would build up when constructing an astrometric reference system
from relative positions of stars within a small field of view on the sky.
In addition to this main detection system, another photomultiplier system detected light from a beam splitter in the optical path to measure the Johnson B- and
V-band photometric magnitudes of another 400 000 stars down to 11th magnitude
and with a positioning accuracy of 50 mas.
Hipparcos revolutionized the field of astrometry. It vastly improved our knowledge of stellar distances out to about 100 pc and allowed statistical studies of
stellar properties with much higher precision than had been possible by any previous ground-based observations.
The results of the past ten years of exploitation of the Hipparcos satellite data
are comprehensively summarized by Perryman (2009).
COBE
– Launch: 18 November 1989, Delta from Vandenberg AFB, USA
– Orbit: 900 km LEO
– Instruments: Differential Microwave Radiometer (DMR), Far-InfraRed Absolute
Spectrophotometer (FIRAS), Diffuse InfraRed Background Experiment (DIRBE)
– Mass: 2200 kg
– Lifetime: 4.1 a
COBE was designed to measure the diffuse radiation from the Cosmos between
1 µm and 1 cm wavelength over the entire sky. Its main mission was to see if
the Cosmic Microwave Background closely followed a Planck function at 2.7 K,
as expected from the Big Bang cosmological theory. COBE then revealed subtle
spatial variations in the cosmic background radiation of one part in 105 across the
sky. In addition, the short wavelength detectors of the DIRBE instrument observed
the diffuse infrared background radiation between wavelengths of 1 µm and 300 µm.
Thus COBE also provided a wide-field sky survey at near-infrared wavelengths.
COBE scanned the sky by rotating 1 min−1 about its symmetry axis, oriented at
94 to the Sun-Earth line. It covered the entire sky every six months in this manner,
allowing redundant measurements of every direction to reduce uncertainties.
◦
The two principal investigators for COBE instruments, John Mather and George
Smoot, were awarded the 2006 Nobel Prize for physics for their discoveries with
COBE.
The instrument and its performance — two years after launch — has been described by Boggess et al (1992) and the results of all four years of observations
were reported by Bennett et al (1996).
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6. The visible and near-infrared domain
HST
– Launch: 25 April 1990, Space Shuttle from Cape Canaveral, USA
– Orbit: 690 km circular low Earth orbit (LEO)
– Optical system: 2.4 m f /24 Ritchey–Chrétien telescope
– Wavelength range: 0.115 µm to 2.5 µm
– Instruments: cameras, spectrometers and interferometers that are exchanged during visits by astronauts
– Mass: 11600 kg
– Lifetime: ongoing 19 a after launch
The HST was one of NASA’s four flagship missions called the Great Observatories and was a joint NASA/ESA collaboration both for construction and operation.
It was designed as an astronomical observatory to provide diffraction-limited images from ultraviolet to near-infrared wavelengths using a suite of instruments at
different spectral resolutions and fields of view.
Launched in 1990, its performance during the initial three years in orbit was
hampered owing to a misfigured primary mirror. New instruments installed in 1993
on the first servicing mission corrected this flaw, allowing Hubble to become one of
the most productive and best known of all space missions. It remains operational
through 2009; another servicing mission is planned for 2009 to extend the mission.
Hubble employs a number of novel technologies allowing it to achieve its full
resolution and sensitivity. Chief among these are a set of six gyroscopes with unprecedented accuracy and an interferometric Fine Guidance System that uses stars
to stabilize the pointing. Together, these subsystems allow the telescope to point
anywhere on the sky with an RMS jitter of less than 5 mas. HST employs a number of innovative subsystems, including six nickel-hydrogen batteries to power the
observatory during the half-hour period in Earth shadow every orbit, and it was
the first observatory satellite to use the TDRSS communication system for its
telemetry.
Hubble has contributed to nearly every field of astrophysics and many in solar
system research as well. To explore the harvest from HST one should consult the
web site http://www.stsci.edu/hst/.
IRTS
– Launch: 13 March 1995 by an HII rocket from the Tanagashima Space Center,
Japan, returned by the Space Shuttle
– Orbit: LEO
– Optical system: 15 cm liquid helium cooled telescope
– Wavelength range: 1 µm to 1000 µm
– Instruments: Near-InfraRed Spectrometer (NIRS; 1.4 µm to 4.0 µm), Mid-Infrared
Spectrometer (MIRS; 4.5 µm to 11.7 µm), Far-Infrared Line Mapper (FILM; 145 µm,
155 µm, 158 µm, 160 µm), Far-InfraRed Photometer (FIRP; 150 µm, 250 µm,
400 µm, 700 µm)
– Lifetime: 20 d
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IRTS was one of the early satellites launched by the Japanese space agency,
JAXA, designed primarily to carry out an infrared spectroscopic survey of 7 %
of the sky and provide spectral energy distributions of the objects at far-infrared
wavelengths using the FIRP instrument. IRTS had a small telescope and short
lifetime, thus representing a first step as JAXA entered into the more recent era of
large aperture space telescopes for astronomical research (Murakami et al 1996).
ISO
– Launch: 17 November 1995, Ariane 4 from Kourou, French Guiana
– Orbit: 1000 km to 70 500 km elliptical (period 24 h; of which ca. 17 h outside the
radiation belts)
– Optical system: 0.6 m telescope cooled with superfluid helium
– Wavelength range: 2.5 µm to 240 µm
– Instruments: two spectrometers (SWS and LWS), a camera (ISOCAM) and an
imaging photo-polarimeter (ISOPHOT) jointly covered wavelengths from 2.5 µm
to around 240 µm
– Mass: 1800 kg (launch mass, including the liquid helium, 2400 kg)
– Lifetime: 28 months
The European Space Agency’s ISO was the first true orbiting infrared observatory. ISO was a follow-on to the IRAS survey and a precursor to the Spitzer
observatory. ISO’s 60 cm telescope was cooled with superfluid helium to allow natural background-limited sensitivity at far infrared wavelengths. It had a field of
view of 20′ and stabilized pointing to an accuracy of 5′′ .
ISO was an important advance over IRAS in space observations of the infrared
sky. It observed many sources discovered earlier to ascertain their luminous output,
chemical composition, structure, and nature. ISO was especially important for its
spectroscopy of the interstellar medium in the Milky Way and many distant galaxies, advancing our understanding of infrared luminous galaxies undergoing bursts of
star formation. It showed spectra of circumstellar regions around young stars that
matched those of solar system objects, establishing the chemical similarity of these
regions — and it demonstrated that water is present everywhere in the Universe.
The ISO science legacy is summarized in a series of papers edited by Cesarsky and
Salama (2005).
MSX
– Launch: 24 April 1996, Delta II from Vandenberg AFB, USA
– Orbit: 900 km, polar, near-Sun synchronous
– Wavelength range: 4.2 µm to 26 µm
– Instruments: SPIRIT III, a five-colour, high-spatial resolution scanning radiometer and a six-channel high-spectral resolution Fourier-transform spectrometer; UVISI,
five spectrographic imagers and four UV/visible imagers; and Space-Based Visible,
a 16 cm visible-band imaging telescope
– Mass: 2700 kg
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6. The visible and near-infrared domain
– Lifetime: 10 months
The MSX was a military test project sponsored by the Ballistic Missile Defense
Organization (BMDO) to demonstrate the feasibility of identifying and tracking
ballistic missiles midway through their trajectories. It employed three instruments
covering the range 0.11 µm to 28 µm with multispectral capability producing data
cubes that combine spatial and spectral information. MSX also carried out some
aeronomic and auroral observations for civilian use, in addition to its military
mission (cf., Mill and Guilmain 1996).
Spitzer
– Launch: 25 August 2003, Delta 7920H ELV from Cape Canaveral, USA
– Orbit: Earth-trailing, heliocentric
– Optical system: 0.85 m telescope cooled with liquid helium
– Wavelength range: 3 µm to 180 µm
– Instruments: four-channel array camera (IRAC), imaging photometer (MPS),
low-resolution spectrometer (IRS)
– Mass: 865 kg
– Lifetime: ≥ 5 a
Spitzer, previously known as SIRTF, is a pointed general-purpose infrared telescope similar to ISO built by NASA as the fourth of its ‘Great Observatories’
programme.
The 0.85 m Ritchey–Chrétien telescope is cooled with liquid helium to reduce
thermal backgrounds. Its five-year lifetime is made possible by using radiative cooling of its outer shell to reduce the heat load on the cryogen tank, and it uses an
Earth-trailing heliocentric orbit to further reduce thermal loads from the Earth and
provide a high field of regard. Spitzer ’s pointing system is based on celestial-inertial
three-axis stabilized control.
The three infrared instruments provide a combination of imaging and spectroscopy: IRAC is a four-channel camera at (3.6, 4.5, 5.8, and 8) µm; MPS is a
multi-channel photometer with detectors at (24, 70, and 160) µm; IRS is a spectrometer providing continuous coverage between 5 µm and 40 µm. Spitzer remains
operational at the time of writing.
Spitzer has followed on to the science of IRAS and ISO by vastly increasing
our understanding of distant galaxies, especially those luminous in the infrared, the
interstellar medium, circumstellar regions, especially those with the precursors to
planetary systems, and extra-solar planetary systems (Werner et al 2004, and all
following articles in the same volume). One especially noteworthy result achieved
from observations with Spitzer was the detection of the thermal radiation from an
extra-solar planet for the first time, putting constraints on the planet’s temperature
and size (Charbonneau et al 2005).
127
COROT
– Launch: 27 December 2006 by a Soyuz-Fregat launcher from Baikonur, Kazakhstan
– Orbit: 896 km circular polar orbit, allowing continuous observations of two regions in the sky for more than 150 days each
– Optical system: 0.27 m telescope
– Instrument: a wide-field (2.8◦ × 2.8◦ ) two-part camera operating in the visible —
one for each mission objective, namely exoplanet search and asteroseismology
– Mass: 630 kg
– Lifetime: nominally 2.5 a
COROT, a mission led by the French Space Agency (CNES), with contributions
from ESA, Austria, Belgium, Germany, Spain and Brazil, uses a wide-field telescope
that is designed to detect tiny changes in the brightness of nearby stars. The
mission’s main objectives are the search for exoplanets by detecting the dimming
of the light from a star as a planet passes in front of it, as well as the study of
stellar interiors by the method of asteroseismology, with the aim to calculate the
star’s precise mass, age, and chemical composition (Auvergne et al 2009).
COROT is the first mission capable of detecting rocky planets that are several
times larger than Earth, around nearby stars. In spring 2008, COROT discovered
planet COROT-Exo-7b,whose passage in front of the stellar disk dims the star
to 99.65 % of its normal brightness (Léger et al 2009). Based on this observation
in space, supplemented by observations from the ground, it was concluded that
COROT-Exo-7b, which orbits a K0V star (T = 5300 K) within 20 h, was a truly
Earth-like, “rocky” planet. Given its surface temperature of over 1000 ◦ C, however,
the surface most likely is covered by lava.
Bibliography
Allen CW, Cox AN (2000) Astrophysical quantities. Berlin: Springer
Auvergne M, Bodin P, Boisnard L, Buey J-T, Chaintreuil S, CoRoT
team (2009) The CoRoT satellite in flight: description and performance:
2009arXiv0901.2206A, DOI: 10.1051/0004-6361/200810860
Beckwith SVW, Stiavelli M, Koekemoer AM (plus 12 authors) (2006) The Hubble
ultra deep field. Astrophys J 132:1729–1755
Bennett CL, Banday AJ, Górski KM (plus seven authors) (1996) Four-year COBE 1
DMR Cosmic Microwave Background observations: Maps and basic results. Astrophys J 464:L1–L4
Bernstein RA, Freedman WL, Madore BF (2002) The first detections of the extragalactic background light at 3000, 5500, and 8000 Å. I. Results. Astrophys J
571:107–128
Boggess N W, Mather J C, Weiss R (plus 15 authors) (1992) The COBE mission —
Its design and performance two years after launch. Astrophys J 397:420–429
Brown TM, Ferguson HC, Smith E (plus four authors) (2004) Age constraints for
an M31 globular cluster from main-sequence photometry. Astrophys J 613:L125–
L128
128
6. The visible and near-infrared domain
Cesarsky CJ, Salama A, eds (2005) ISO science legacy. Space Sci Rev 119:Nos 1–4
Charbonneau D, Allen LE, Megeath ST (plus eight authors) (2005) Detection of
thermal emission from an extrasolar planet. Astrophys J 626:523–529
Charbonneau D, Brown TM, Noyes RW, Gilliland RL (2002) Detection of an extrasolar planet atmosphere. Astrophys J 568:377–384
Freedman WL, Madore BF, Gibson BK (plus 12 authors) (2001) Final results
from the Hubble Space Telescope key project to measure the Hubble constant.
Astrophys J 553:47–72
Franceschini A, Aussel H, Bressan A (plus seven authors) (1997) Source-counts and
background radiation. ESA SP-401:159–167
Fröhlich C (2010) Solar radiometry. ISSI SR-009:525–540
Ginzburg VL, Syrovatskii SI (1965) Cosmic magnetobremsstrahlung (Synchrotron
radiation). Ann Rev Astron Astrophys 3:297–350
Gruber DE, Matteson JL, Peterson LE, Jung GV (1999) The spectrum of diffuse
cosmic hard X-rays measured with HEAO 1. Astrophys J 520:124–129
Hardy JW (1998) Adaptive optics for astronomical telescopes. Oxford University
Press
Horch E, Heanue JF, Morgan JS, Timothy JG (1994) Speckle imaging with the
MAMA detector: Preliminary results. Astron Soc Pacific Publ 106:992–1002
Hauser MG, Dwek E (2001) The cosmic infrared background: Measurements and
implications. Ann Rev Astron Astrophys 39:249–307
Léger A, Puget JL (1984) Identification of the “unidentified” IR emission features
of interstellar dust? Astron Astrophys 137:L5–L8
Léger A, Rouan D, Schneider J (2009) Transiting exoplanets from the CoRoT space
mission — VIII. CoRoT-7b: The first super-Earth with measured radius. Astron.
Astrophys. (submitted for publication in a special issue with ca. 80 CoRoT papers)
Lindegren L (2010) High-accuracy positioning: astrometry. ISSI SR-009:279–291
Madau P, Pozzetti L (2000) Deep galaxy counts, extragalactic background light
and the stellar baryon budget. Mon Not R astr Soc 312:L9–L15
Mathis JS (1990) Interstellar dust and extinction. Ann Rev Astron Astrophys
28:37–70
Mill JD, Guilmain BD (1996) The MSX mission objectives. Johns Hopkins Appl
Phys Lab Tech Dig 17:4–10
Murakami H, Freund MM, Ganga K (plus 25 authors) (1996) The IRTS (Infrared
Telescope in Space) mission. Publ Astron Soc Japan 48:L41-L46
Neugebauer G, Soifer BT, Beichman CA (plus seven authors) (1984) Science
224:14–21
Perryman MAC (2009) Astronomical applications of astrometry: A review based on
ten years of exploitation of the Hipparcos satellite data. Cambridge: Cambridge
University Press
Puget JL, Léger A (1989) A new component of the interstellar matter — Small
grains and large aromatic molecules. Ann Rev Astron Astrophys 27:161–198
Riess AG, Strolger L-G, Tonry J (plus 16 authors) (2004) Type Ia supernova discoveries at z > 1 from the Hubble Space Telescope: Evidence for past deceleration
and constraints on dark energy evolution. Astrophys J 607:665–687
129
Sellgren K (1984) The near-infrared continuum emission of visual reflection nebulae.
Astrophys J 277:623–633
Spitzer L (1978) Physical processes in the interstellar medium. New York: WileyInterscience
Stenflo JO (2010) Stokes polarimetry of the Zeeman and Hanle effects. ISSI SR009:543–557
Stanev T, Franceschini A (1998) Constraints on the extragalactic infrared background from gamma-ray observations of MRK 501. Astrophys J 494:L159–L162
Title AM (2010) Michelson interferometers. ISSI SR-009:327–338
van de Hulst HC (1981) Light scattering by small particles. New York: Dover
van der Marel RP, de Zeeuw PT, Rix H-W, Quinlan GD (1997) A massive black
hole at the centre of the quiescent galaxy M32. Nature 385:610–612
Williams RE, Blacker B, Dickinson M (plus 14 authors) (1996) The Hubble deep
field: Observations, data reduction, and galaxy photometry. Astron J 112:1335–
1384
Werner MW, Roellig TL, Low FJ (plus 25 authors) (2004) The Spitzer Space
Telescope mission. Astrophys J Suppl 154:1-9
Wright EL (1996) On the density of primordial black holes in the galactic halo.
Astrophys J 459:487–490
Wright EL (2008) personal communication